The present invention relates to a method of driving a recording apparatus and, more particularly, it relates to a driving method at the time of driving a liquid crystal in a time division manner.
The recording apparatus in which a liquid crystal light shutter is employed is intended to carry out light writing on a photosensitive body by driving opened or closed a plurality of micro shutters in the liquid crystal light shutter by means of the control circuit, to shield or transmit the light of a light source arranged adjacent to the liquid crystal light shutter. In the case of this type of recording apparatus, the liquid crystal is required to have high speed response. Therefore, a liquid crystal whose dielectric anisotropy is inverted by the frequency of a signal applied to the micro shutters is employed and driven by two frequencies, one (f.sub.H) of which is higher than a frequency (f.sub.C) which makes the dielectric anisotropy of the liquid crystal zero, and the other (f.sub.L) of which is lower than the frequency (f.sub.C) In the case of such liquid crystal light shutter, recording is attained at a density of about 10 dots per 1 mm and in the case of size A4, for example, the micro shutters of about 3,000 units are needed per one line. For the purpose of preventing the number of wires and mounting area from being increased, therefore, the liquid crystal light shutter is usually driven in a time division manner. This time division drive is of such type that common and signal electrodes are crossed perpendicular to one another in the liquid crystal light shutter to form micro shutters at the crossed portions of both electrodes and recording signals are inputted to the signal electrodes while time-divided selection signals are inputted to the common electrodes.
According to this time division drive or 2-time division drive, for example, the light can be transmitted only for half the writing period T.sub.W, and the time period during which the light can be transmitted becomes shorter in the case of n-time division drive, so that the photosensitive body can be short of the amount of exposure light. Therefore, the drive is performed in such a way that the selection signals are used to open or close the micro shutters for a selected time period T.sub.W /n of the one writing period T.sub.W and keep them set for the remaining time period (1-1/n)T.sub.W (which will be hereinafter referred to as non-selected time period) of the one writing period T.sub.W. In the case of 2-time division drive, for example, selected signal in which signals of f.sub.H, f.sub.L, *f.sub.H and *f.sub.L shown in FIG. 1 A , said signals of *f.sub.H and *f.sub.L being shifted by 180.degree. in phase from said signals of f.sub.H and f.sub.L, mixed in one writing cycle is formed and applied to the common electrode. Further, one of four recording signals 1-4 shown in FIG. 1 B is selected and applied to the signal electrodes. Four kinds of drive signals 5-8 shown in FIG. 1 C are thus formed and one of these drive signals is applied to the micro shutters to thereby drive them opened or closed. Since a signal which is shifted by T.sub.W /2 in phase from the signal which has been applied to the above-mentioned common electrode is applied to the other common electrode, a drive signal corresponding to the one shown in FIG. 1 C but shifted by T.sub.W /2 in phase is applied to the micro shutters to drive them opened or closed.
Although the liquid crystal light shutter which is used to explain the present invention have been formed by a liquid crystal of the GH type, for example, which allows light to be transmitted through it upon its on-operation, a liquid crystal of the TN type which is provided between polarizing plates positioned at an orthogonal Nicol prism and which shields light upon its on-operation may be employed but it should be understood that the liquid crystal light shutter in this case is opened or closed reversely to those of the GH type liquid crystal.
Light transmission characteristics of the micro shutters obtained when the drive signals 5-8 are applied to the micro shutters are shown in FIG. 1D. A light transmission characteristic 9 is due to the drive signal 5 which is obtained when the pattern signal 1 is applied to the signal electrodes while the selected signal shown in FIG. 1 A is applied to the common electrodes, with the result that the paired micro shutters are opened. A light transmission characteristic 10 is due to the drive signal 6 which is obtained when the pattern signal 2 is applied to the signal electrodes while the selected signal shown in FIG. 1 A is applied to the common electrodes, with the result that ones of the paired micro shutters are open and the others are closed. Similarly, light transmission characteristics 11 and 12 are due to the drive signals 7 and 8 which are obtained when the pattern signals 3 and 4 are applied to close ones of the micro shutters and open the others in such a manner as to be reverse to the above-mentioned case, or close both of these micro shutters. (f.sub.L - f.sub.H) shown in FIG. 1 C represents a drive signal formed by the signals of f.sub.L and f.sub.H, (f.sub.H - *f.sub.H), a drive signal formed by the signals of f.sub.L and *f.sub.H and (0) a silent signal.
When the liquid crystal light shutter are driven like this, selected micro shutters can be kept open even for the non-selected time period (1-1/n)T.sub.W in the case of n-time division drive and the signal f.sub.L is applied to the micro shutters for the last time period of one writing cycle T.sub.W to open them and eliminate the hysteresis effect peculiar to the liquid crystal, thereby enabling the liquid crystal light shutters to be opened or closed as if they were driven according to the static drive.
In the case of the above-described time division drive for the liquid crystal light shutter, however, the liquid crystal light shutters can be kept under their previously-set state for a non-selected time period, using the hysteresis effect of the liquid crystal, but their optimum temperature ranges only from 2 to 3 degrees. In addition, their optimum temperature changes, depending upon the quality of liquid crystal used, and it was therefore necessary to set a different temperature range in every recording apparatus. Further, the amount of light transmitted at the time of shutter opening is less sufficient compared to the case where the shutters are opened according to the static drive.
As apparent from the level difference between the light transmission characteristics 11 and 12, the amount of light transmitted when the micro shutters are closed becomes different, depending upon whether ones of the paired micro shutters are kept opened or closed at the time when the others are closed, thereby making the density of a recorded image different. This difference in the amount of light transmitted is caused more or less within the above-mentioned optimum temperature range and it becomes larger outside the optimum temperature range. In order to reduce the difference in the amount of light transmission, it is proposed that the voltage value of a frequency signal which serves to drive the liquid crystal employed by the recording apparatus is changed, but a more complicated circuit for obtaining multiple voltage values of this signal is needed.